In a triangle ABC AB = BC, the external angle with angle C is 125 degrees. Find angle B.

It is known from the condition that in triangle ABC the sides AB = BC, and the external angle with the angle С is equal to 125 °. We need to find the degree measure of the angle B.

The triangle that is given to us is isosceles, since the condition says that AB = BC.

We need to find the angle at the vertex B, and the angles at the base are equal to each other.

Let us first of all apply the inside angle at the vertex C, if it is known that the sum of adjacent angles is 180 °.

180 ° – 125 ° = 55 °.

The base angles are 55 °.

The sum of the angles of the triangle, we are looking for the angle at the vertex:

180 ° – (55 ° – 55 °) = 70 °.